Many rubber articles, principally automobile tires, but also including hoses, conveyor belts, power train belts, e.g., transmission belts, and the like, are usually reinforced with fibrous or metal cords. In all such instances, the fiber must be firmly bonded to the rubber. This is so whether the fiber is a natural or synthetic polymer, or metallic, and whether the rubbers are natural or synthetic.
The conventional practice has been to prepare the fiber by pretreatment with a combination of hexamethoxymelamine or hexamethylene-tetramine and phenol-formaldehyde condensation product, wherein the phenol is almost always resorcinol. By a mechanism not completely understood, the resin reacts with the fiber and the rubber, effecting a firm reinforcing bond.
One method for preparing rubber compositions reinforced with cords entails compounding a vulcanizing rubber stock composition with the components of an adhesive resin condensation product. The components of the condensation product include a methylene acceptor and a methylene donor. The most commonly employed methylene acceptor is a phenol, such as resorcinol, while the most commonly employed methylene donor is a melamine, such as N-(substituted oxymethyl)melamine. The effect achieved is resin formation in-situ during vulcanization of the rubber, creating a bond between the metal or polymeric cords and the rubber, irrespective of whether the cords have been pretreated with an additional adhesive, such as a styrene-butadiene latex, polyepoxides with a blocked isocyanate, and the like.
Resorcinol-free vulcanizable rubber compositions are known. For example, U.S. Pat. No. 5,298,539 discloses vulcanizable rubber compositions containing uncured rubber, a vulcanizing agent and at least one additive selected from the group consisting of derivatives of melamine, acetoguanamine, benzoguanamine, cyclohexylguanamine and glycoluril monomer and oligomers of these monomers. These derivatives are substituted on average at two or more positions on the monomer or each unit of the oligomer with vinyl terminated radicals and the composition is free of resorcinol.
Another manner of eliminating resorcinol from vulcanizable rubber compositions has relied on the use of alternative coreactants. U.S. Pat. No. 4,038,220 describes a vulcanizable rubber composition which comprises a rubber, a filler material, N-(substituted oxymethyl)melamine and at least one of α- or β-naphthol. This reference employs the monohydric phenols, α- or β-naphthol, as methylene acceptors in the resin forming reaction during vulcanization in the absence of resorcinol. The use of resorcinol-formaldehyde resin to replace resorcinol in vulcanizable rubber compositions is also known. For example, see A. Peterson, et al., “Resorcinol Bonding Systems for Steel Cord Adhesion”, Rubber World (August 1984).
An increased need in the industry for fiber reinforcing of rubber to survive high dynamic stress, such as flexing, to avoid tire belt separation has brought about a continuing search for other and better methods for achieving high adhesive strength.
Tires typically have a construction such that a carcass, edge portions of a belt, an under-belt pad and the like are intricately combined with each other in its shoulder portion. The under-belt pad provided continuously along the circumferential shoulder portion of the tire between a tread rubber portion and the carcass and extending outwardly of the belt edge portions along the width of the tire is a thick rubber layer, which is a structural characteristic for alleviating a shear stress possibly generated between the belt edge portions and the carcass. Further, since the under-belt pad is repeatedly subjected to loads during running, heat is liable to build-up in the under-belt pad, thereby causing internal rubber destruction in the under-belt pad and adhesion failures between the rubber components and between a rubber portion and cords (steel cords) in the carcass. This causes separation of the belt edge portions and ply separation in the carcass resulting in breakdown of the tire. One conventional approach to this problem is that the under-belt pad is formed of a rubber compound which contains a reduced amount of carbon black for suppression of heat build-up.
However, the rubber compound for the under-belt pad is softened by the reduction of the carbon black content therein. This also results in the adhesion failure and the internal rubber destruction in the under-belt pad due to the heat build-up, thereby causing the ply separation and the belt separation in the tire during running. Therefore, this approach is not satisfactory in terms of the durability of the tire. The deterioration of the durability of the tire which results from the heat build-up attributable to the structural characteristic of the under-belt pad is a more critical problem, since the recent performance improvement of automobiles requires that tires have a higher durability under higher speed running and heavy loads.
Despite their good abrasion resistance, radial tires become unusable sooner than bias tires because of the belt separation which takes place while the tread still remains. One way that this problem has been addressed is by improving the tread or steel cord-embedding rubber. For example, an improved tread is of dual layer structure, with the inner layer (base tread adjacent to the belt) being made of a rubber composition which is saved from heat generation at the sacrifice of abrasion resistance, and the outer layer (cap tread) being made of a rubber composition of high abrasion resistance. Also, an improved steel cord-embedding rubber is made of a rubber composition containing an adhesive such as a cobalt salt of an organic acid, hydroxybenzoic acid, and resorcinol, which increases adhesion between rubber and steel cord. These improvements, however, are not completely successful.
Other adhesion promoters have been used in an attempt to avoid belt separation, for example, special latices such as, for example, a vinyl-pyridine latex (VP latex) which is a copolymer of about 70% butadiene, about 15% styrene and about 15% 2-vinylpyridine. Examples of other latices which are present in adhesion promoters are acrylonitrile rubber latices or styrene-butadiene rubber latices. These can be used as such or in combination with one another. Especially suitable adhesion promoters for polyesters are also those which are applied in multi-stage processes, for instance a blocked isocyanate being applied in combination with polyepoxide and the material then being treated using customary resorcinol-formaldehyde resins (RFL dip). It is also known to use combinations of RFL dips with other adhesion-promoting substances such as, for example, a reaction product of triallyl cyanurate, resorcinol and formaldehyde or p-chlorophenol, resorcinol and formaldehyde.
Not only is it necessary that adhesion between rubber and metal, e.g., steel or polymeric cord be high, but it is also necessary that a decrease in adhesion be as small as possible while tires are in use. In actuality, tires containing a steel cord-embedding rubber with good adhesion occasionally lose the initial adhesion to a great extent after use. The following are the possible reasons why adhesion between steel cord and rubber decreases while tires are in use:                (1) Tires are subject to many minute cuts when they run over gravel or sharp objects. The cuts reaching the inside of the tread permit air and moisture to infiltrate into the tire, promoting the aging and fatigue of the embedding rubber and also rusting the steel cord. All this leads to a decrease in adhesion.        (2) The adhesion improver incorporated into the steel cord-embedding rubber diffuses and migrates into the tread rubber during vulcanization or tire use. This leads to a decrease in adhesion.        (3) The softener and other additives incorporated into the tread migrate into the steel cord-embedding rubber. This also leads to a decrease in adhesion.        
Despite the various proposals made to improve the adherence of cord to rubber in vulcanizable rubber compositions, there is a continuing need for commercially available cost effective additives that improve the adhesion of rubber to fibrous or metal cords in vulcanizable rubber compositions.
The adhesion promoter systems of the present invention far surpass any extant adhesion promoters known in the art for adhesion of metal and/or polymeric cord to vulcanizable rubber.